Manifestations of the post-thrombotic syndrome, including pain, edema, hyperpigmentation, and ulceration, may develop in as many as two-thirds of patients after an episode of deep venous thrombosis (DVT). The hemodynamics of this syndrome have been well characterized and are related to the development of venous valvular incompetence. However, such sequelae are not universal and may be preventable in some patients. We now know that the venous lumen is eventually re-established in the majority of patients, although recurrent thrombotic events are also common. Early data suggests that the rate of recanalization, the occurrence of recurrent thrombotic events, and the persistence of residual venous obstruction may influence the development of venous valvular incompetence. However, the determinants of these events in individual patients, as well as the impact of therapeutic measures such as anticoagulation and thrombolytic therapy are unknown. The goals of this study are to determine the relationship between the rates of recanalization and recurrent thrombotic events and the ultimate development of valvular incompetence after an episode of acute DVT. This will be accomplished using serial:duplex ultrasound examination of the lower extremities at intervals of 1, 3, 7, and 14 days; 1, 3, 6, and 9 months; and 1, 2, and 3 years after the acute event. The occurrence of these events will then be related to the underlying clinical risk factors, laboratory markers of coagulation and fibrinolysis, and duration and intensity of anticoagulation in an effort to identify groups at risk for delayed recanalization, recurrent thrombosis, and ultimate development of valvular incompetence. Such information is critical in defining an appropriate duration of anticoagulation based upon risk of rethrombosis as well as in identifying subpopulations that might benefit from measures such as thrombolytic therapy based upon their risk of inefficient recanalization. Improved direction of such interventions would presumably reduce the incidence of valvular incompetence and subsequent post-thrombotic syndrome.